In recent years, there has been a growing interest in a technique of supplying electric power in a wireless manner without using a cable, that is, in a contactless manner, to an electric instrument or an electric device movable without being fixedly positioned, such as a mobile phone, a personal digital assistance (PDA), an electrically assisted bicycle, an electric vehicle, a hybrid automobile, etc. For instance, Japanese Unexamined Patent Application Publication No. 2011-234496 (Patent Document 1) discloses a technique for contactless power supply to a vehicle from a power source disposed outside the vehicle. Also, Japanese Unexamined Patent Application Publication No. 2008-244575 (Patent Document 2) discloses a technique relating to an antenna coil for short-distance wireless communication.
In this way, in a coil unit including a resonance coil (antenna coil) incorporated in a resonance circuit to function as an antenna, in order to suppress adverse influence of magnetic field generated in its circumference to other devices or to provide the antenna with a desired directivity for transmission/reception, it is generally implemented to provide a shield on the side for shielding the magnetic field. However, disposing a shield can sometimes be undesirable if disposing space or cost needs to be taken into consideration.
The shield-less coil unit disclosed in Patent Document 1 includes a plurality of resonance coils and these coils are disposed such that the magnetic field generated in one resonance coil and the magnetic field generated in at least one other resonance coil are in opposite phases. With this arrangement, the magnetic fields in the other location than the resonance circuits may be cancelled out each other, thereby to reduce leakage magnetic field (Patent Document 1: paragraphs 5-12, FIG. 1, FIG. 2, etc.). However, with such cancellation of magnetic fields, there occurs reduction in the strength of the magnetic field for a same amount of power supplied to the power supplying side resonance coil, so that reduction in power supply efficiency too occurs.
Further, in the case of the antenna coil for short-distance wireless communication disclosed in Patent Document 2, instead of a plate-like shield, the coil employs, as a shield, a loop coil provided separately from a resonance oil acting as an antenna. And, this loop coil is disposed at a position interlinked with the non-communication direction side components of the magnetic flux generated by the resonance coil acting as the antenna. As a result, the antenna coil can be switched over between a state having high directivity and a state having weak directivity for allowing communication over a wider area, by switching over between a state when a loop of the loop coil is formed and a state when the loop is disconnected (Patent Document 2: paragraphs 31, 48-50, etc.). With addition of a shield in the form of a plate-like shield or a loop coil, the directivity on the side where the shield is present is restricted, so that the directivity on the side where the shield is absent can be enhanced relatively. However, the addition of shield may lead to wasteful consumption of energy and deterioration in transmission efficiency.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2011-234496
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2008-244575